PWM generates 150 or 300 volts

Versions of this high-voltage generator circuit without programmable frequency have replaced variacs in motor-speed controllers and ATE equipment. Another version uses a step-down transformer for testing very high-current circuit breakers. This one generates 150 or 300 V, and 1.5 kW.

In local terms, the SA03 pulse-width modulator (PWM) is running open loop, but overall operation is closed loop by virtue of the system computer, which monitors performance and makes adjustments per calibration tables and correction algorithms.

The 1-V peak output of the MAX038 waveform generator is stepped up to 10 V peak going into the multiplying D/A converter. The summing amplifier is scaled for maximum peak output of 2 V and is offset about 5 V. The scaling for the dc correction signal is about plus/minus 250 mV. The ac signal jumper allows master or slave operation of the module.

First-order theory only dictates that the power transformer should have more than enough inductance to do all the filtering. Cores used for low-frequency power do not work well at all with 22-kHz square waves, so some filtering is required. Using 450 microhenries sets the pole at 435 Hz and will keep 22-kHz ripple current below 1.2 A peak. This may need adjustment, depending on the specific power transformer. The split primary allows current monitor signals containing very little ac common-mode voltage.

There are four secondaries. With a slave module importing the ac signal from the master, the two amplifiers will be in phase at the signal frequency even though they may not be in phase at the switching frequency.

Power doubling is achieved by adding at the transformer stage rather than actually paralleling the PWM amplifiers. Frequency and magnitude are controlled by the master only, but the slave does use its own dc correction loop. The master/slave approach allows interchangeable modules in 750-W and 1,500-W test systems.